A rotorcraft is provided with at least one engine that drives the main rotor for the purpose of providing the vehicle with lift and also with propulsion. The rotor comprises a hub that is set into motion by a rotor shaft driven by the engine, and on which a plurality of blades are arranged.
As they rotate, the blades are subjected to a torser of forces, and consequently they are subjected to centrifugal force and also to multiple forces due to flapping, drag, and twisting, where twisting is caused in particular by changes of pitch for the purpose of altering the angle of inclination of the blade relative to the plane of the hub.
In addition, the hub generally has a plurality of arms, possibly flexible in flapping, with the blades being arranged at the ends of the arms. The forces due in particular to centrifugal force are then transmitted to the non-flexible central zone of the hub by the spar of the blade.
A first type of blade is known that is fitted with a spar extending from the root of the blade along the span of the blade and being arranged at the leading edge of the blade. That type of spar is referred to as a “leading-edge spar” for convenience in the text below.
That type of spar is effective, but appears to be insufficient for modern blades that present shapes that are complex, e.g. for acoustic reasons.
A second type of blade is known that has a spar referred to as a distributed spar. Such a spar consists in a leading-edge spar that is extended by two portions placed flat, respectively on the suction side and the pressure side of the blade.
Nevertheless, it is difficult to attach a blade fitted with a distributed spar to a rotor hub.
U.S. Pat. No. 3,923,422 discloses a device for performing such a function. At the root of the blade, the distributed spar is wound around a vertical axis parallel to the axis of rotation of the rotor.
Nevertheless, in order to perform such a vertical winding, it is necessary to twist the portion of the spar that is placed flat on the suction side and on the pressure side of the blade. Such twisting through about ninety degrees presents the drawback of being potentially destructive insofar as the spar is constituted mainly by unidirectional fibers for blades made out of composite material.
It is also difficult to place the unidirectional fibers coming from the twisted section of the distributed spar flat on the suction side and the pressure side of the blade.
Twisting can then lead to poor reproducibility in terms of shape and also in terms of mechanical and vibratory characteristics from one blade to another.
Furthermore, if filler elements need to be arranged in the blade, they will be of complex shape and will vary from one blade to another because the twisting of the spar is not reproduced identically. It thus becomes difficult to automate blade manufacture.
It is then possible to envisage winding the spar “horizontally” in accordance with the proposal described by French patent No. 2 186 380.
Nevertheless, that solution is proposed solely for a spar that is placed flat on the suction side and the pressure side of the blade, but not on the leading edge of the blade. Consequently, that does not constitute a distributed spar but a spar of a completely different type. In addition, French patent No. 2 186 380 does not present any teaching relating to the method that enables horizontal winding to be performed.
Finally, that blade does not include fastener means. It is then arranged in an arm of a rotorcraft hub of a very special shape and is then secured to said arm by means of a pin passing through the arm to lock the spar of the blade where the spar is wound.
Presumably to prevent excessive flapping of the blade, the arm comprises top and bottom plates clamped onto the spar of the blade.
The blade of French patent No. 2 186 380 is thus a specific blade that does not have a distributed spar and that is suitable for fitting only to a special type of rotor hub.